The present invention relates to a method of efficiently forming thin-piece samples for use in a transmission electron microscope (TEM), or the like, using a focused ion beam (FIB).
Section TEM samples are required to be formed so that the desired point of observation is at a thickness through which an electron beam can be transmitted. Conventionally, it has been well known that an ion beam may be used to form a test/examination sample into a desired shape. Forming a sample for use in a transmission electron microscope (TEM) is performed by cutting the sample, using a focused ion beam, to a proper thickness so that electrons can transmit through the sample. This sample forming method is disclosed in the reference entitled, xe2x80x9cSection TEM Sample Forming Method Using a Focused Ion Beamxe2x80x9d, 37th Applied Physical Academy, March 1990. Also, Japanese Patent Application No. 192641/1990 (JP-A-4-76437), filed by the present applicant, is also related to this technology. When a sample is machine-polished down to approximately several 10 xcexcm, it is further cut at opposite surfaces of an observational point by ion beam etching so as to leave a thin wall of less than 1 xcexcm, which makes it more convenient to confirm and observe a forming position, forming shape, section, etc. The prior application discloses an invention where a scanning electron microscope irradiating an electron beam is arranged so as to form a sample while monitoring a forming portion of the sample. Furthermore, disclosure is also made of the technology where the sample surface is locally formed with a film so as to prevent any damage due to the ion beam, and the inclination angle of a forming surface resulting from a convergent angle of the ion beam is compensated for by using an inclination setting in a table holding the sample in order to produce evenness in the sample thickness.
A summary of the basic technology of a focused ion beam forming apparatus for the present invention will be explained referring to an embodiment of the prior application shown in FIG. 5.
When a sample 4 to be formed is placed on a stage 5, the sample chamber is placed in a vacuum state by a vacuum apparatus (not shown), The sample stage 5 is set to a desired positional angle by a drive mechanism (not shown). The drive mechanism, in general, is capable of: (1) displacement in the X, Y and Z directions, (2) ion beam axis rotation, and (3) adjustment in angle relative to an ion beam axis. When a forming region is determined, a region of the sample having an end portion of a thin wall is exposed to a chemical vapor deposition (CVD) gas from the gas gun 9 in order to prevent damage to the portion by the ion beam, and a metal protection film is formed. Next, the forming region is irradiated by an ion beam and cut by sputtering. In this case, the relative displacement of the ion beam 2 and the sample 4 is made by scanning the ion beam with the deflection member of the electrostatic optical system 3, without using a drive device, because the forming requires extreme precision on the order of microns. Initially, the ion beam current is adjusted to increase the sputter rate in order to shorten the process time, thereby performing rough forming. Finally, the ion current is decreased in the area of the sample forming region, thereby performing precision forming. The feature of this apparatus lies in a structure where an electron beam can be irradiated to a sample for sample surface observation in a direction different than that of the ion beam. Because of this arrangement, an electron beam 7 may be solely used for scanning a sample that would have been damaged by an ion beam. The secondary charged particles (electrons) 11 are then detected by the secondary charged particle detector 10. Thus, observations may be made at the same time the samples are formed without taking the samples out of the apparatus. Also, because scanning electron microscope (SEM) images and scanning ion microscope (SIM) images are different due to the different kinds of secondary charged particles 11 emitted from the sample, images having different resolutions may be obtained. Accordingly, both images can be compared side-by-side on a display 13.
FIGS. 2A to 2D refer to the forming of a sample for use in a transmission electron microscope (TEM). A holding piece (not shown) is fixed with a sample block 4 that is mechanically cut out and is placed on a sample table (stage) 5 of the focused ion beam apparatus through a holder (not shown), and an ion beam 2 is irradiated to form the sample (see also FIG. 5). The specific procedure is as follows. In the first step, as shown in FIG. 2A, a sample 4 is a sectionally convex-formed block formed by mechanical cutting, and the sample is placed on the sample stage 5 (see FIG. 5). Next, a forming frame of the sample is determined, and, in order to prevent damage by the irradiating ion beam to the end portion being formed into a thin wall, a CVD gas (e.g., phenanthrene C14H10) is applied to that portion, thereby forming a protective coating layer 41 (as shown in FIG. 2B). In the next step, an ion beam 2 is irradiated and the sample block is cut at opposite surfaces by sputtering. Thus, a thin wall 42 of a sample becomes formed as shown in FIG. 2C. This sample for use in a transmission electron microscope has no differences in the shape of the opposite surfaces, and the sample is required to be of a thickness where an incoming electron beam 7 to the thin wall from a perpendicular direction can transmit through the thin wall (being 0.5 xcexcm or less), as shown in FIG. 2D.
When reducing the thickness of a sample by using an ion beam, the material at the opposite surfaces of a predetermined region on the sample block is sputtered out, thereby making a desired thin wall 42 (as shown in FIG. 2C). The ion beam is scanned in a raster-like fashion to gradually cut the wall surface. As shown in FIG. 1A, the main scanning is made in a wall width direction (X direction), while sub-scanning is made in a wall thickness direction (Y direction). The directions of the main scanning and sub-scanning are generally built into the ion beam forming apparatus, and the formation of the thin wall on one side of the sample block is first performed by cutting along the surface from an outer wall and gradually advancing deeper into the sample block. On the other side of the sample block, an ion beam is irradiated into a predetermined position in the interior of the sample block, and the sample block on the other side is cut inside-out from the sample block. That is, the hole width is gradually increased in an outer-wall direction, and finally the outer wall is cut out in process.
As shown in FIG. 3, the cutting speed for a silicon substrate sputtering with an ion beam has specific incident angle characteristics. More specifically, a value of 0 is given for a cutting speed at a beam incident angle of 90 degrees to the surface to be formed, because the beam cannot be irradiated onto the sample surface. However, as the ion beam is inclined a little to enable irradiation to the sample surface, the cutting speed abruptly increases to peak out at about 80 degrees. As the angle of the ion beam is adjusted, the forming efficiency gradually decreases to the value at 0 degrees, which is an angle of an incident beam in the perpendicular direction to the surface to be formed, wherein the characteristic in cutting speed is about xe2x85x9 that of the peak value. The positional relationship between the sample and the ion beam to be irradiated, as can be understood from FIG. 2C and FIGS. 4A and 4B, is maximized for the greatest efficiency at an incident angle of approximately 80 degrees. However, when forming a hole in parallel with the inner end wall surface (that is, cutting the sample block from the inside-out), the forming is naturally started at a lower efficiency rating because the incident ion beam angle is initially at 0 degrees (that is, the beam is directly perpendicular to the sample surface). When cutting the sample block from the outer wall inwards, the sample material that is cut out by sputtering is scattered to the outside of the sample block and away from the interior of the sample block. However, when the sample block is cut from the inside-out by cutting a hole parallel with the inner end wall surface, the sample material that is sputtered remains inside the hole, some of which re-adheres (see FIG. 4B, 44) to the bottom or adjacent wall surface of the sample block being cut. Therefore, when the sample block is cut from the inside-out, multiple passes with the ion beam must be performed in order to properly cut out the sample block. Although the time required to make multiple-passes with the ion beam is not a significant obstacle when only forming one sample a day, it does become a problem when 5 to 6 samples are required to be formed in a single day.
The present invention is directed to a method of forming a thin-piece sample for use in an electron microscope. The ion beam scanning used for etching a sample block to form a thin-wall portion is initiated from the outer perimeter of two opposite sides of the sample block to be formed, one side at a time, and the ion beam is directed from the outer perimeter of the sample block inwards towards the center of the sample block. When the two sides of the sample block are etched from the outside into the sample block, a thin wall is produced at the interior portion of the sample block.
Also, a plurality of samples may be set in a known positional relationship, and a series of forming functions, including ion beam scanning, may be programmed for automation, allowing a plurality of samples to be formed all at one time easily and efficiently.
FIG. 1A is a top view from an ion beam source of conventional beam scanning directions relative to a sample;
FIG. 1B is a top view from an ion beam source of beam scanning directions according to an embodiment of the present invention;
FIG. 2A is a perspective view of a sample block formed by mechanical cutting;
FIG. 2B is a perspective view showing a metal protection film formed at the thin-wall end portion using a gas gun and then irradiated by an ion beam;
FIG. 2C is a perspective view of a sample formed into a thin piece by an ion beam;
FIG. 2D is a perspective view showing a positional relationship between a created TEM sample and a transmission electron beam;
FIG. 3 illustrates a graph showing a relationship between an etch rate and an incident angle of the ion beam;
FIG. 4A is a side view showing a focused ion beam forming of the sample block from an outer wall surface side of the sample block;
FIG. 4B is a view showing a focused ion beam forming of the sample block from the inside-out of the sample block; and
FIG. 5 is a schematic view showing an example of an ion beam forming apparatus.